Intervertebral disc pathology can be the result of many factors including injury, aging, environmental factors, tumors, infection, and genetics. Intervertebral disc pathology can result in the absence of physiological loading of vertebral end plates resulting in instability or degenerative changes over time, which may lead to spinal stenosis and neurological complications.
Several surgical techniques have been developed to address intervertebral disc pathology and associated diseases that affect the verbal endplates, to which the discs transmit their load. Spinal decompression with or without disc removal and fusion has become a recognized surgical procedure for mitigating spinal column pain by restoring biomechanical and anatomical integrity to the spine. Spinal fusion is recommended based on a variety of clinical indications. Fusion techniques may involve the excision of intervertebral disc material and the preparation of the disc space for receiving an implant to aid in fusion and transmission of the load from vertebrae and maintain vertebral column shape after the fusion process. The surgically-placed implants (spacers) can rest on the exposed vertebral endplates.
Spinal fusion procedures are generally conducted using a posterior or an anterior approach. Anterior cervical inter-body fusion (ACDF) procedures generally have the advantages of reduced operative times, lower infection rate, and reduced blood loss. Further, anterior procedures do not interfere with the posterior anatomic structure of the spine. Anterior procedures also minimize scarring within the spinal canal and are advantageous from a structural and biomechanical perspective. The generally preferred anterior procedures are particularly advantageous in providing improved access to the disc space, and correspondingly better endplate preparation.
Several inter-body implant systems have been introduced to facilitate inter-body fusion. Traditional threaded implants or cages, of varying shapes and material, are typically packed with bone graft material and surgically placed in the intervertebral disc space. However, a relatively small portion of the vertebral endplate is in contacted with these implants. These implant bodies often engage the softer cancellous bone in the center of the vertebra, rather than the stronger cortical bone, the uncinate process, or the apophyseal rim of the vertebral endplate. The seating of threaded cylindrical implants may also compromise biomechanical integrity by reducing the area in which to distribute mechanical forces, thus increasing the apparent stress experienced by both the implant and vertebrae. Further, a substantial uncontrolled risk of implant subsidence (defined as sinking or settling) into the softer cancellous bone of the vertebral body may arise from such improper seating.
Even open ring-shaped cage or spacer implant systems, generally shaped to mimic the anatomical contour of the vertebral body, lack the ability to complement specific stiffness of the patient's bone. Traditional ring-shaped cages are generally comprised of allograft bone material, harvested from the human donors. Such allograft bone material restricts the usable size and shape of the resultant implant. For example, many of these ring-shaped bones generally have a medial-lateral width of less than 25 mm for the lumbar spine and 14 mm for cervical spine. Therefore, these allograft cages may not be of a sufficient size to contact the strong cortical bone, the uncinate process, or apophyseal rim of the vertebral endplate. These size-limited implant systems may also poorly accommodate related instrumentation such as drivers, reamers, distractors, and the like. For example, these implant systems may lack sufficient structural integrity to withstand repeated impact and may fracture during implantation. Further, other traditional non-allograft ring-shaped cage systems may be size-limited due to various and complex supplemental implant instrumentation, which may obstruct the disc space while requiring greater exposure of the operative field. These supplemental implant instrumentation systems also generally increase the instrument load on the surgeon.
The surgical procedure corresponding to an implant system should preserve as much vertebral endplate bone surface as possible by minimizing the amount of bone removed. This vertebral endplate bone surface, or subchondral bone, is generally much stronger than the underlying cancellous bone. Preservation of the endplate bone stock ensures biomechanical integrity of the endplates and minimizes the risk of implant subsidence. Thus, proper interbody implant design should provide for optimal seating of the implant while utilizing the maximum amount of available supporting vertebral bone stock.
Traditional interbody spinal implants generally do not seat properly on the preferred structural bone located near the apophyseal rim of the vertebral body, which is primarily composed of preferred dense subchondral bone. Accordingly, there is a need in the art for interbody spinal implants which better utilize the structurally supportive bone of the apophyseal rim.
In summary, separate challenges can be identified as inherent in traditional anterior P spinal fusion devices: 1) end-plate preparation; 2) implant retention; 3) implant subsidence; 4) bone graft volume; 5) implant incorporation with vertebral bone; and 6) radiographic visualization.
1. End-Plate Preparation
There are three traditional end-plate preparation methods. The first is aggressive end-plate removal with box chisel-types of tools to create a match between end-plate geometry and implant geometry. In the process of aggressive end-plate removal, however, the end-plates are typically destroyed. Such destruction means that the load-bearing implant is pressed against soft cancellous bone increasing the risk of implant subsidence.
The second traditional end-plate preparation method preserves the end-plates by just removing cartilage with curettes. The end-plates are concave; hence, if a flat implant is used, the interface will not be well matched and the implant may not be very stable. Even if a convex implant is used, it is very difficult to match the implant geometry with the end-plate geometry, as the end-plate geometry varies from patient-to-patient and on the extent of disease.
The third but lesser used, traditional end-plate preparation method uses threaded fusion cages. The cages are implanted by burring out corresponding threads in the end-plates. This method also violates the structure.
2. Implant Retention
Traditional implants can migrate and expel out of the intervertebral body space following the path through which the implant was inserted. Typical implants are either threaded into place or have large “teeth” designed to prevent expulsion. Both options can create localized stress risers in the end-plates, increasing the chances of subsidence. The challenge of preventing implant expulsion is especially acute for PEEK implants, because the surface texture of PEEK is co very smooth and slippery, with reduced purchase on the adjacent vertebrae.
3. Implant Subsidence
Subsidence of the implant is a complex issue and has been attributed to many factors. Some of these factors include aggressive removal of the endplate; an implant stiffness significantly greater than the vertebral bone; smaller sized implants which tend to sit in the center of the disc space against the weakest region of the end-plates; and implants with sharp edges which can cause localized stress fractures in the end-plates at the point of contact. The most common solution to the problem of subsidence is to choose a less stiff implant material. This is why PEEK and cadaver bone have become the most common materials for spinal fusion implants. PEEK is less stiff than cortical bone, but more stiff than cancellous bone. PEEK is a preferred choice for loading bone graft within an implant. In accordance with Wolfe's Law, the bone graft within the implant should be loaded in order for it to convert to living bone tissue. Living bone bridging from one vertebral body through the spacer and joining with the second vertebral body is the definition of “interbody fusion” which is one the primary goals of an ACDF procedure.
4. Bone Graft Volume
Cadaver bone implants are restricted in their size by the bone from which they are machined. Their wall thickness also must be great to create sufficient structural integrity for their desired clinical application. These design restrictions do not leave much room for filling the bone graft material into cortical bone implants. The exposure-driven limitations on implant size narrow the room left inside the implant geometry for bone grafting even for metal implants. Such room is further reduced in the case of PEEK implants because their wall thickness needs to be greater compared to metal implants due to structural integrity requirements.
5. Incorporation with Vertebral Bone
In many cases, the typical interbody fusion implant is not able to incorporate with the vertebral bone, even years after implantation. Such inability persists despite the use of a variety of different materials to construct the implants. PEEK has been reported to become surrounded by fibrous tissue which precludes it from incorporating with surrounding bone. Stiff, typically metallic, implants stress shield the bone graft and do not supports its transformation into living bone. In some designs of metal implants, such as those made of commercially pure titanium and titanium alloy, or tantalum and tantalum alloys, have surfaces that allow for bone ingrowth or on-growth and in some case even stimulate bone formation.
6. Limitations on Radiographic Visualization
For implants made of metal, the metal limits adequate radiographic visualization of the bone graft. Hence it can be difficult to assess fusion, if it is intended to take place. PEEK is radiolucent, so traditional implants made of PEEK need to have radiographic markers embedded into the implants so that implant position can be tracked on an X-ray. Cadaver bone has some radiopacity and does not interfere with radiographic assessment as much as metal implants. Metal implants are dense and inhibit the assessment of boney fusion via x-ray techniques. In addition, they can create significant artifacts when utilizing MRI or CT scans to post-operatively visualize the implant/bone interfaces.
Therefore, a need exists for improvements to interbody implants and the present invention is directed to cure such need.